This invention relates to electric generator field direct current (DC) exciters for industrial turbine-generator assemblies and, in particular, to techniques for replacing a brushless exciter (BE) with a static DC excitation system.
Many large industrial turbine generator systems, such as those generating more than 100 MegaWatts (>100 MW) use a brushless exciter as the power source for the DC current required for the electric generator electro-magnet field winding. This DC power source is commonly referred to as the generator excitation system or simply the “exciter”.
The field windings of the generator are typically mounted on a rotor that is surrounded by a stator. The exciter is electrically connected to the field windings on the rotor. The DC current from the exciter in the rotor field windings forms a magnetic field around the rotor. A turbine rotationally drives the rotor. As the magnetic field from the rotor field windings rotates through the stator, voltage is induced in the stator windings. The current in the stator is output as power by the generator.
Brushless exciters have had a history of unexpected failures and expensive repairs. As they age, brushless exciters tend to fail more often and their failures become more severe. As the demand for electricity grows, the demands on the utility interconnection electrical grids are requiring generator excitation systems with faster dynamic characteristics and higher power. To address these issues, electric power producers often elect to replace their brushless excitation systems with a static excitation system.
Static excitation systems are well-known and proven technologies for providing DC current to a field windings of a generator rotor in larger generator turbine assembly. A static excitation system typically includes a DC power supply and a mechanical assembly (often referred to as a “collector”) that connects to the rotor of the generator. It is known how to replace a brushless excitation system with a static excitation system.
To replace a brushless exciter, the collector of the static excitation system must be matched to the turbine generator rotor system. Matching the collector to the turbine generator rotor system includes designing the collector such that it has the same mechanical dynamic and vibration characteristics at the coupling to the generator rotor as did the brushless exciter. Typically, the engineer designing the collector has extensive information regarding the structure, mechanical dynamic characteristics and vibration characteristics of the turbine generator rotor system.
Situations do arise where the engineer designing the collector does not have extensive information regarding the turbine generator rotor system. Matching the mechanical, dynamic and vibration characteristics of the brushless exciter is problematic if detailed information on the structure and design of the turbine generator rotor system is not available. To determine the impact the smaller collector rotor will have on the rotor dynamic characteristics of the turbine generator rotor system that originally had a brushless exciter, detailed design information is required for the system which typically is only available to the original equipment manufacturer (OEM). Various ways exist to determine the rotor dynamic characteristics of the system but they are costly. For example, much of the needed mechanical information about the turbine generator rotor system may be obtained by disassembly, inspection and measurement of the components of the turbine generator rotor system. Disassembly of the turbine generator rotor system requires that the generator turbine assembly be taken off-line and stopped.
It is generally not economical to take off-line a turbine-generator system to measure and collect data on the turbine generator rotor system. There is a long felt need for a system and method to design a collector to replace a brushless exciter without detailed existing documentation describing the turbine generator rotor system and while the brushless exciter and its associated turbine-generator continue to operate and produce electrical power.